Usb 6356

We used a thin and flexible resistive pressure sensor (1 lb, 9.5-mm diameter, A201, FlexiForce, Tekscan) for probe pressure measurement [Fig. 1(a)]. To calibrate the sensor for absolute pressure measurement, we used a series of custom built metal weights with a contact surface area equivalent to effective area of the pressure sensor (i.e., $0.71{\mathrm{cm}}^2$ ), as shown in Fig. 1(b). During calibration, the pressure sensor was sandwiched between metal weights to ensure exact overlap of the sensor area with the weights’ contact surface area [Fig. 1(c)]. The output voltage of the sensor (stabilized with a voltage follower circuit²⁷ ) was recorded with a DAQ board (USB-6356, NI). Different weights (between 20 and 335 g) were employed to apply 21 pressures against the sensor, which ranged from 20.7 to 346.3 mmHg. Note three repetitions of the calibration measurement were made at each applied pressure, and the mean of the sensor’s output voltage across the three repetitions was computed. This mean output voltage from the sensor was plotted against the exact probe pressure exerted by the weights, and a linear best-fit line was computed [Fig. 1(d)]. This linear best-fit line was then used to convert voltage from the sensor to absolute probe pressure on tissue.